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                <p><img src="https://jack-1307599355.cos.ap-shanghai.myqcloud.com/9f27cb5c78ed96868b74263320fc5dcd.png" alt="Linux之crontab命令"></p>
<h1 id="Linux周期任务：at和crontab"><a href="#Linux周期任务：at和crontab" class="headerlink" title="Linux周期任务：at和crontab"></a>Linux周期任务：at和crontab</h1><p>每个人或多或少都有一些约会或者是工作，有的工作是长期周期性的， 例如：</p>
<ul>
<li>每个月一次的工作报告</li>
<li>每周一次的午餐会报</li>
<li>每天需要的打卡</li>
<li>……</li>
</ul>
<p>有的工作则是一次性临时的，例如：</p>
<ul>
<li>刚好课题组老师演讲，需要你明天准备演讲器材等等</li>
<li>临时和师妹晚上约了吃饭</li>
<li>……</li>
</ul>
<p>同理，<code>Linux</code>中也有很多周期性任务或者临时任务，例如：</p>
<ul>
<li>每周打包一份备份文件：每周一的凌晨 3:25 把/home/wwwroot 目录打包备份为 backup.tar.gz。</li>
<li>今晚 23:30 重启网站服务</li>
<li>……</li>
</ul>
<p>为此，<code>Linux</code>中提供了<code>at</code>和<code>crontab</code>两个程序来完成一次性临时任务和长期周期任务。</p>
<h2 id="1-一次性临时任务：at"><a href="#1-一次性临时任务：at" class="headerlink" title="1. 一次性临时任务：at"></a>1. 一次性临时任务：at</h2><p><strong>at命令</strong> 用于在指定时间执行命令。</p>
<h3 id="A-启动atd"><a href="#A-启动atd" class="headerlink" title="A. 启动atd"></a>A. 启动atd</h3><p>为了能够在指定的时间执行命令，需要有一个守护进程在后台不断地运行，而后到指定的时间之后运行我们指定的程序。这个守护进程就是 <code>atd</code>。因此，为了使用<code>at</code>程序，需要先开启 <code>atd</code> 守护进程。<code>Linux</code>中使用 <code>systemctl</code> 来管理所有的守护进程，包括启动、重启、禁止、……</p>
<p>首先开启 <code>atd</code> 守护进程启动那个服务</p>
<pre class="line-numbers language-bash"><code class="language-bash"><span class="token function">sudo</span> systemctl <span class="token function">enable</span> rpc-statd
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>然后启动一下 atd 守护进程（默认是启动的，这里是以防万一）</p>
<pre class="line-numbers language-shell"><code class="language-shell">sudo systemctl start atd
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>最后查看一下 atd 守护进程的状态</p>
<pre class="line-numbers language-shell"><code class="language-shell">systemctl status atd
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p><img src="https://jack-1307599355.cos.ap-shanghai.myqcloud.com/image-20230716213941654.png" alt="查看atd守护进程的状态"></p>
<p>可以看到正常在运行</p>
<h3 id="B-at配置文件"><a href="#B-at配置文件" class="headerlink" title="B. at配置文件"></a>B. at配置文件</h3><p><code>atd</code> 通过两个文件 <code>/etc/at.allow</code> 和 <code>/etc/at.deny</code> 来决定系统中哪些用户可以使用 <code>at</code> 设置定时任务</p>
<ul>
<li>它首先检查 <code>/etc/at.allow</code>，如果文件存在，则只有文件中列出的用户（每行一个用户名）能使用 <code>at</code></li>
<li>如果 <code>/etc/at.allow</code>不存在，则检查文件 <code>/etc/at.deny</code>，不在此文件中的所有用户都可以使用 <code>at</code></li>
<li>如果 <code>/etc/at.deny</code> 是空文件，则表示系统中所有用户都可以使用 <code>at</code></li>
<li>如果 <code>/etc/at.deny</code> 文件也不存在，则只有<code>root</code>才能使用<code>at</code></li>
</ul>
<h3 id="C-时间指定格式"><a href="#C-时间指定格式" class="headerlink" title="C. 时间指定格式"></a>C. 时间指定格式</h3><p><code>at</code>允许使用一套相当复杂的指定时间的方法。</p>
<ul>
<li>它能够接受在当天的<code>hh:mm</code>（<code>小时:分钟</code>）式的时间指定。假如该时间已过去，那么就放在第二天执行。</li>
<li>当然也能够使用<code>midnight</code>（深夜）、<code>noon</code>（中午）、<code>teatime</code>（饮茶时间，一般是下午4点）等比较模糊的词语来指定时间。</li>
<li>用户还能够采用12小时计时制，即在时间后面加上<code>AM</code>（上午）或<code>PM</code>（下午）来说明是上午还是下午。 </li>
<li>也能够指定命令执行的具体日期，指定格式为<code>month day</code>（<code>月 日</code>）或<code>mm/dd/yy</code>（<code>月/日/年</code>）或<code>dd.mm.yy</code>（<code>日.月.年</code>）。指定的日期必须跟在指定时间的后面。</li>
</ul>
<p>不过一般不会使用这种绝对时间写法，而是使用相对时间写法。这对于安排不久就要执行的命令是很有好处的。</p>
<p>指定格式为：</p>
<pre class="line-numbers language-shell"><code class="language-shell">now + count time-units
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<ul>
<li><code>now</code>就是当前时间</li>
<li><code>time-units</code>是时间单位，这里能够是<code>minutes</code>（分钟）、<code>hours</code>（小时）、<code>days</code>（天）、<code>weeks</code>（星期）。</li>
<li><code>count</code>是时间的数量，究竟是几天，还是几小时</li>
</ul>
<p> 还有有一种计时方法就是直接使用<code>today</code>（今天）、<code>tomorrow</code>（明天）来指定完成命令的时间。</p>
<h3 id="D-语法"><a href="#D-语法" class="headerlink" title="D. 语法"></a>D. 语法</h3><p>at的语法如下：</p>
<pre class="line-numbers language-shell"><code class="language-shell">at <时间> [选项]
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>其中，</p>
<ul>
<li><code>&lt;时间&gt;</code>是指定任务执行时间的参数，可以是具体的时间或者相对时间。</li>
<li>选项用于指定一些额外的设置，例如任务的优先级、环境变量等。</li>
</ul>
<p>一般来说，在输入完时间和选项后，按下回车就进入到<code>at</code>的交互终端了。在这个终端中我们可以输入我们需要执行的命令。完成输入后按下<code>Ctrl</code>+<code>D</code>结束输入</p>
<blockquote>
<p><code>Ctrl+D</code>是<code>Shell</code>输入结束符的方式</p>
</blockquote>
<pre class="line-numbers language-shell"><code class="language-shell">at now+1min
warning: commands will be executed using /bin/sh
at> echo "1 min passed"
at> <EOT>
job 2 at Sun Jul 16 12:51:00 2023
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p><img src="https://jack-1307599355.cos.ap-shanghai.myqcloud.com/image-20230716125113327.png" alt="at输入命令"></p>
<p>另外一种方式就是使用<code>-f</code>参数，然后指定需要运行的脚本文件就行了</p>
<h3 id="C-运行结果"><a href="#C-运行结果" class="headerlink" title="C. 运行结果"></a>C. 运行结果</h3><p><code>at</code>命令默认情况下不会将任务的输出直接发送到命令行。因此，需要我们在<code>at</code>中写的任务的把输出重定向到一个文件中，该文件可以在任务执行完成后进行查看。</p>
<p>不过一般我们更喜欢能够<code>at</code>能够类似于闹钟一样，任务执行完可以以某种方式提醒我们，所以一般直接在任务的最后结合一个<code>mail</code>命令来向我们的邮箱发邮件</p>
<p>注意，通过<code>Linux</code>系统自带的<code>mail</code>命令默认只能给本机的其他用户发邮件，如果需要对公网邮箱发送邮件，需要配置一下。具体来说，<code>mail</code>命令是依赖<code>sendmail</code>的，所以我们需要先检查安装和配置<code>sendmail</code>。一般系统都自带<code>sendmail</code>，但是只能给内网的邮箱发邮件。如果想给公网的邮箱发邮件（比如qq邮箱）就需要配置<code>sendmail</code>。</p>
<p>配置也比较简单，因为发邮件是应用层，所以配置一下<code>SMTP</code>协议的相关信息就行了。</p>
<h3 id="D-选项"><a href="#D-选项" class="headerlink" title="D. 选项"></a>D. 选项</h3><pre class="line-numbers language-shell"><code class="language-shell">-f：指定包含具体指令的任务文件；
-q：指定新任务的队列名称；
-l：显示待执行任务的列表；
-d：删除指定的待执行任务；
-m：任务执行完成后向用户发送E-mail。一般多在任务中用mail命令
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h3 id="E-参数"><a href="#E-参数" class="headerlink" title="E. 参数"></a>E. 参数</h3><p>日期时间：指定任务执行的日期时间。</p>
<h3 id="F-实例"><a href="#F-实例" class="headerlink" title="F. 实例"></a>F. 实例</h3><p>明天17点钟，输出时间到指定文件内：</p>
<pre class="line-numbers language-shell"><code class="language-shell">[root@localhost ~]# at 17:20 tomorrow
at> date >/root/2013.log
at> <EOT>
job 8 at 2013-01-06 17:20
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>计划任务设定后，在没有执行之前我们可以用atq命令来查看系统没有执行工作任务：</p>
<pre class="line-numbers language-shell"><code class="language-shell">[root@localhost ~]# atq
8       2013-01-06 17:20 a root
7       2013-01-08 17:00 a root
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
<p>删除已经设置的任务：</p>
<pre class="line-numbers language-shell"><code class="language-shell">[root@localhost ~]# atq
8       2013-01-06 17:20 a root
7       2013-01-08 17:00 a root

[root@localhost ~]# atrm 7
[root@localhost ~]# atq
8       2013-01-06 17:20 a root
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>显示已经设置的任务内容：</p>
<pre class="line-numbers language-shell"><code class="language-shell">[root@localhost ~]# at -c 8
#!/bin/sh
# atrun uid=0 gid=0
# mail     root 0
umask 22此处省略n个字符
date >/root/2013.log
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h2 id="2-长期周期性任务：crontab"><a href="#2-长期周期性任务：crontab" class="headerlink" title="2. 长期周期性任务：crontab"></a>2. 长期周期性任务：crontab</h2><p><strong><code>crontab</code>命令</strong> 被用来提交和管理用户的需要周期性执行的任务，与<code>windows</code>下的计划任务类似，当安装完成操作系统后，默认会安装此服务工具，并且会自动启动<code>crond</code>进程，<code>crond</code>进程每分钟会定期检查是否有要执行的任务，如果有要执行的任务，则自动执行该任务。</p>
<h3 id="A-语法"><a href="#A-语法" class="headerlink" title="A. 语法"></a>A. 语法</h3><pre class="line-numbers language-bash"><code class="language-bash">crontab<span class="token punctuation">(</span>选项<span class="token punctuation">)</span><span class="token punctuation">(</span>参数<span class="token punctuation">)</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

                
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                        var data_url = data.url;
                        data_url = data_url.indexOf('/') === 0 ? data.url : '/' + data_url;
                        var index_title = -1;
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                            keywords.forEach(function (keyword, i) {
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                });
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});
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